[PATCH v2 1/4] block/mq-deadline: reject zero prio_aging_expire

[Ye Bin <yebin@huaweicloud.com>]

From: Ye Bin <yebin10@huawei.com>

A prio_aging_expire of zero does not disable I/O priority in
mq-deadline.  Instead the priority aging path in
dd_dispatch_prio_aged_requests() is invoked with "now - 0 == now",
which causes best-effort and idle requests to be dispatched ahead of
pending real-time requests -- a classic priority inversion, not the
"priority disabled" behavior users may expect when writing zero.

Reject zero (and negative) values in the sysfs store with -EINVAL so
that a misconfiguration is reported rather than silently accepted.

Signed-off-by: Ye Bin <yebin10@huawei.com>
---
 block/mq-deadline.c | 28 ++++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/block/mq-deadline.c b/block/mq-deadline.c
index 5f643c0ce2a8..a4358625cfc8 100644
--- a/block/mq-deadline.c
+++ b/block/mq-deadline.c
@@ -31,7 +31,10 @@ static const int read_expire = HZ / 2;  /* max time before a read is submitted.
 static const int write_expire = 5 * HZ; /* ditto for writes, these limits are SOFT! */
 /*
  * Time after which to dispatch lower priority requests even if higher
- * priority requests are pending.
+ * priority requests are pending.  Must be > 0: a value of zero would make
+ * the priority aging path dispatch best-effort and idle requests ahead of
+ * pending real-time requests through "now - 0 == now", a classic priority
+ * inversion.
  */
 static const int prio_aging_expire = 10 * HZ;
 static const int writes_starved = 2;    /* max times reads can starve a write */
@@ -770,7 +773,6 @@ static ssize_t __FUNC(struct elevator_queue *e, const char *page, size_t count)
 	STORE_FUNCTION(__FUNC, __PTR, MIN, MAX, msecs_to_jiffies)
 STORE_JIFFIES(deadline_read_expire_store, &dd->fifo_expire[DD_READ], 0, INT_MAX);
 STORE_JIFFIES(deadline_write_expire_store, &dd->fifo_expire[DD_WRITE], 0, INT_MAX);
-STORE_JIFFIES(deadline_prio_aging_expire_store, &dd->prio_aging_expire, 0, INT_MAX);
 STORE_INT(deadline_writes_starved_store, &dd->writes_starved, INT_MIN, INT_MAX);
 STORE_INT(deadline_front_merges_store, &dd->front_merges, 0, 1);
 STORE_INT(deadline_fifo_batch_store, &dd->fifo_batch, 0, INT_MAX);
@@ -778,6 +780,28 @@ STORE_INT(deadline_fifo_batch_store, &dd->fifo_batch, 0, INT_MAX);
 #undef STORE_INT
 #undef STORE_JIFFIES
 
+/*
+ * prio_aging_expire must be positive: a value of zero would make the
+ * priority aging path dispatch best-effort and idle requests ahead of
+ * pending real-time requests through "now - 0 == now", a classic priority
+ * inversion.  Reject zero and negative values instead of clamping, so that
+ * a misconfiguration is reported rather than silently accepted.
+ */
+static ssize_t deadline_prio_aging_expire_store(struct elevator_queue *e,
+						 const char *page, size_t count)
+{
+	struct deadline_data *dd = e->elevator_data;
+	int val, ret;
+
+	ret = kstrtoint(page, 0, &val);
+	if (ret < 0)
+		return ret;
+	if (val <= 0)
+		return -EINVAL;
+	dd->prio_aging_expire = msecs_to_jiffies(val);
+	return count;
+}
+
 #define DD_ATTR(name) \
 	__ATTR(name, 0644, deadline_##name##_show, deadline_##name##_store)
 
-- 
2.34.1